1. Field of the Invention
The present invention relates to semiconductor device manufacture and, more particularly, to methods and structures for attaching semiconductor devices to lead frames.
2. State of the Art
During the manufacture of semiconductor devices, a semi-conductor device or chip is often attached to a support, such as a lead frame. In general, a xe2x80x9cleads-over-diexe2x80x9d or xe2x80x9clead-on-chipxe2x80x9d semiconductor device assembly has a lead frame with lead fingers that extend over the active surface of the semiconductor device supporting the semiconductor device being electrically connected to the bond pads located thereon, typically by wire bonds extending between the bond pads and the ends of the lead fingers, the other ends of the lead fingers being for attachment to other circuit components. The lead fingers extend inwardly on the lead frame to bond pads located on the active surface of the semiconductor device for connection thereto as described in U.S. Pat. No. 4,862,245 (Pashby, et al.) and U.S. Pat. No. 5,304,842 (Farnworth, et al.).
The semiconductor device may be attached to the lead frame using adhesively coated tape as described in U.S. Pat. No. 5,304,842 (Farnworth et al.) or by the use of hot or cold adhesives as described in U.S. Pat. No. 5,286,679 (Farnworth et al.). Use of hot or cold thermosetting or thermoplastic adhesives that have heretofore been suggested affect the processing of the semiconductor device assemblies as they require cure time for the adhesive and, in turn, cause delays in the manufacturing process to effect curing.
Therefore, it is desirable to have an adhesive for attaching the semiconductor device to a lead frame which has the shortest desirable cure time for the adhesive to allow the manufacturing process to proceed.
A system for attaching a semiconductor device or die to each lead frame of a plurality of lead frames includes indexing means for supplying and advancing the plurality of lead frames for semiconductor devices in a lead frame-by-lead frame sequence. Each lead frame of the plurality of lead frames has an attaching surface to which a semiconductor device or die is to be attached. The system also includes a source of curable adhesive. Application means are configured for receiving the plurality of lead frames for semiconductor devices in the lead frame-by-lead frame sequence. The application means is connected to the source of curable adhesive for receiving curable adhesive therefrom. The application means is also configured for applying a metered amount of the curable adhesive in a preselected pattern to the application surface of each lead frame of the plurality of lead frames. The application means then supplies the lead frames with the curable adhesive applied thereto.
The system also includes a source of semiconductor devices to supply semiconductor devices in a semiconductor device-by-semiconductor device sequence. Attaching means are positioned relative to the source of semiconductor devices to obtain each semiconductor device in the semiconductor device-by-semiconductor device sequence. The attaching means is also positioned to receive the lead frames with the curable adhesive applied thereto in lead frame-by-lead frame sequence from the application means. The attaching means is also configured to attach one of the semiconductor devices to a corresponding lead frame of the plurality of lead frames in lead frame-by-lead frame sequence by urging the device into contact with the curable adhesive of each lead frame of the plurality of lead frames and holding each of the semiconductor devices in contact with the curable adhesive for a preselected period. That is, at least one semiconductor device is attached to each lead frame. Of course, in some applications, multiple devices may be attached to a particular lead frame configured to receive multiple semiconductor devices. Control means are provided in the system to supply operation signals to operate the various components thereof.
The attaching means preferably includes press means for pressing each semiconductor device into contact with a curable adhesive. The attaching means also preferably includes transfer means for transferring each semiconductor device in semiconductor device-by-semiconductor device sequence from the source of semiconductor devices to the press means.
The press means desirably includes heating means to heat the semiconductor device before it is pressed against the curable adhesive. The heating means is desirably a block positioned to receive each semiconductor device from the transfer means. The press means most preferably includes a press mechanism to move the block from a receiving position to receive thereon a semiconductor device into an attached position in which the block with a semiconductor device is urged into contact with the curable adhesive. The heating means desirably heats the block to a temperature from about 200xc2x0 C. to about 225xc2x0 C.
The curable adhesive is most preferably a snap curable epoxy having a cure time of substantially less than one minute and most preferably having a cure time of less than one second. In a preferable configuration, it is desirable that the curable adhesive have a filler material included therein. The filler material may be any electrochemically neutral material but is preferably a granulated Teflon(copyright).
The application means is configured to apply about one milligram of the curable adhesive to each lead frame. The application means may also include a structure positioned to contact the application surface of a lead frame for applying the curable adhesive thereto. The application means may include a roller having portions positioned to selectively contact the application surface of the lead frame and the opposing surface. Alternately, the application means may include a printing structure to contact the application surface of the lead frame or the active surface of the semiconductor device. The printing structure may include a plate, a silkscreen die, or the like. The application means may also optionally include one or more nozzles positioned to deposit the curable adhesive in a desired location on either the lead frame or the active surface of a semiconductor device.
In alternate configurations, the lead frames of the plurality of lead frames are connected one to another. The lead frames preferably have at least one and, preferably two, removable edges or rails with drive perforations formed therein. Indexing means includes an electrical device connected to a drive structure which is configured to engage the perforations formed in each lead frame.
The attaching means preferably includes an anvil sized for positioning opposite the block and spaced therefrom to receive each lead frame of the plurality of lead frames therebetween. The anvil functions to support each lead frame as the semiconductor device is pressed against the application surface. An anvil may also be positioned opposite the application means to support each lead frame as the curable adhesive is applied thereto.
A method for applying curable adhesive to each lead frame of a plurality of lead frames and attaching a semiconductor device to each lead frame includes providing a system and operating the system to supply semiconductor devices and lead frames relative to application means and attaching means. The system is also operated to apply adhesive to a semiconductor device site of each lead frame and to then attach a semiconductor device to the adhesive at the semiconductor device site. Preferably, the adhesive is a snap curable epoxy with a cure time of about one (1) second. Even more preferably, the application means includes a pressing structure which includes a block that heats the semiconductor devices to a temperature from about 200 degrees centigrade to about 225 degrees centigrade.